The invention relates to the determination of the quality of an optical instrument and particularly the measurement of its optical transfer function or OTF.
The quality of an optical system is typically represented either by the value of its optical transfer function OTF (in polychromatic light when the instrument is not for operating on a particular wavelength) or by the aberrant deviation .DELTA.(x,y) for each of the points of the pupil as defined by its Cartesian coordinates x and y.
When designing a new optical instrument, the transfer function resulting from its optical formula is computed. It is however necessary to measure if the optical quality of the actual instrument corresponds to the design calculation based on modelization. Numerous methods are already known for determining the optical quality of an existing instrument. All have drawbacks, namely limitations in use which make them inapplicable in numerous cases.
Interferometry methods are known. Reference may be made in that respect to "OTF measurements with a white light source: an interferometric technique" in Applied Optics, Vol. 14, No. 7, July 75, pp. 1613-1615, or "Progress in Optics", Vol. 5, No. 5, 1966, pp. 204-209 and 234-237. They are difficult to put into practice in polychromatic light. Methods are also known for the direct measurement of the MTF, particularly that used in the apparatus sold by the assignees of the present invention, under the tradename "ACOFAM". This apparatus gives satisfactory results for instruments having current dimensions, but it requires a generator and an analyzer whose dimensions are comparable to those of the respective beams. In practice, that means that an instrument of very large or very small size would require a very specific object generator since this generator must be sized to the dimensions of the object field.
Finally Cotton and Hartmann methods are known for measuring the value of the aberrant deviation. For that, a differential method is used consisting in disposing in the pupil of the instrument to be studied a diaphragm pierced with two openings. The Cotton assembly is the one shown schematically in FIG. 1. A pinpoint source 10, monochromatic or at least having a narrow spectrum, illuminates a diaphragm 12 placed in the pupil of the instrument 14 to be studied. The mutually coherent secondary sources formed by the two openings 16 of diaphragm 12 create in the image plane 18 a system of interference fringes. The measurement of the position of the center of the fringes gives an indication about the aberrant deviation difference between the points of the pupil corresponding to the openings of the diaphragm.
The measurement is repeated for several positions of the diaphragm along the directions x and y and gives, by a method of finite increments, a chart of the slopes of the wave surface. The assembly which has just been described appears as directly linked to the geometric optics, whereas the optical transfer function is linked to the diffraction, i.e. to undulatory optical phenomena. It is difficult and laborious to use the slopes of the wave surface to arrive at the optical transfer function, for an integration must be carried out which is all the less strict since using finite increments.